fix mix typo in README.md
$ mix phx.serve
** (Mix) The task "phx.serve" could not be found. Did you mean "phx.server"?
$ mix phx.server
[info] Running Pleroma.Web.Endpoint with Cowboy using http://0.0.0.0:4000
$ mix phx.serve
** (Mix) The task "phx.serve" could not be found. Did you mean "phx.server"?
$ mix phx.server
[info] Running Pleroma.Web.Endpoint with Cowboy using http://0.0.0.0:4000